Proceedings of the 12 th International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012 15B Seagrasses and seagrass ecosystems Establishing tropical seagrass light requirements in a dynamic port environment Kathryn M. Chartrand 1 , Michael Rasheed 1 , Katherina Petrou 2 , Peter Ralph 2 1 Marine Ecology Group, Fisheries Queensland, Cairns QLD 4870 Australia 2 Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, NSW 2007 Australia Corresponding author: Katie.Chartrand@qld.gov.au Abstract. Tropical seagrasses inhabit naturally turbid waters with dynamic light environments and variable water quality in coastal waters adjacent to the Great Barrier Reef. Large tidal fluxes amplify the magnitude of these conditions with extreme high and low light over relatively short time scales (i.e. hours). Large port developments in the region have the potential to confound the complex relationships between seagrass physiology and this dynamic light field with the onset of dredging and their associated turbid plumes. Understanding the capacity for seagrasses to respond to changes in the quantity and quality of the light environment will allow for prediction of how seagrass species and populations will tolerate changes in light attenuation that may occur during dredging. We present a strategy for determining seasonal-specific light requirements for an intertidal tropical seagrass community in a port environment. Locally relevant light requirements are established by describing the relationships among photosynthetic inputs and losses, tidal exposure, shifts in spectral light quality, seasonality and the capacity to utilise below ground carbon reserves. The outcomes of the study provide guidelines for a mitigation strategy that is focused on maintaining critical windows of light to support seagrass growth and the longer term survival of these productive coastal ecosystems. Key words: Seagrass, Light Requirements, Management. Introduction One of the major drivers of seagrass growth and distribution in shallow coastal environments worldwide is light availability (Dennison 1987, Duarte 1991, Ralph et al. 2007). However, naturally turbid inshore waters related to coastal runoff, large tidal fluxes and complex hydrodynamics can create narrow windows of opportunity for photosynthetic gains and seagrass viability. Seagrasses adapted to these marginal environments can be particularly sensitive to reductions in light beyond this established range of conditions. On Queensland’s east coast, seagrasses cover approximately 38,079 sq km of inshore habitat, within the boundary of the Great Barrier Reef Marine Park (GBRMP; Fig. 1; McKenzie et al. 2010). These coastal seagrass meadows support the coral reef ecosystem due to the many roles they play including: acting as a nursery for a wide range of fisheries; providing feeding grounds for predatory fish and invertebrates; nutrient filtering from coastal runoff, sediment trapping from riverine catchments and; export of organic matter to adjacent and downstream systems (Watson et al. 1993, Hemminga and Duarte 2000, Heck et al. 2008). In Australia, human-induced changes to the inshore environment have been documented to cause seagrass loss largely due to light limitation (Walker and McComb 1992, Ralph et al. 2006). Along the Queensland east coast, anthropogenic pressures and the associated risk to seagrasses can be high in areas where urban development and most notably port infrastructure has a strong foothold (Grech et al. 2011). Along the GBR, seagrass meadows are common in close proximity to large port facilities creating conflicts between economic development and ecological success of the habitat. Currently, there is a push for rapid expansion of new and established ports along the Queensland coast to fulfill a greater demand for mineral- and gas exports to overseas markets (Fig. 1; BREE 2011). The capital works to expand wharves and swing berths puts additional pressure on adjacent seagrass meadows with threats to water quality, (most importantly light availability), from large scale dredging programs and associated turbidity plumes. Seagrasses have been shown to work well as bio- indicators for assessing the light availability and overall health status of marine coastal environments (Dennison et al., 1993, Abal and Dennison 1996). Government regulators have called for an ecological based approach to managing dredging impacts and